The physiological bases of the sexual dimorphism which affects reproductive behavior are poorly understood. This project will study the molecular mechanisms by which testosterone (T) activates male sexual behavior and will especially focus on the sex differences in copulatory behavior and their neuroendocrine control within the preoptic area (POA). The quail will be used as model for these studies because its copulatory behavior shows an extreme sexual dimorphism and its hormonal and social environments can be strictly controlled during ontogeny (as a precocial bird it can be raised in isolation, embryos are not influenced by the hormones of their siblings or their mother and can be easily treated with hormones in the egg). The activation of copulatory behavior by T implies its partial transformation into estrogens under the action of the enzyme aromatase. I have already shown that T induces preoptic aromatase activity in a sexually dimorphic manner and increases the staining and volume of the sexually dimorphic medial preoptic nucleus (POM). The discovery of these markers of T action and their sexual dimorphism can enable to make rapid progress in the understanding of T action on behavior despite the great heterogeneity of the POA. More precisely, the objectives of this project are: to determine the exact localization of the sexually dimorphic T-inducible aromatase activity using the Palkovits microdissection technique, to understand the mechanisms of induction of aromatase activity by T (repression of inhibitors, synthesis of new enzyme), to describe it characteristics (hormonal specificity, time and dose response), and to determine the exact role of the aromatase and of the POM in behavioral activation. Experiments will also research now the sexual dimorphism in aromatase inducibility develops during ontogeny and test whether or not this sexual differentiation corresponds to specific changes in cell numbers within the critical areas, especially within the sexually dimorphic POM which probably contains very high levels of aromatase activity.